Wireless charging system with object detection

ABSTRACT

A wireless power transmitting device transmits wireless power signals to a wireless power receiving device. The wireless power receiving device has a wireless power receiving coil in a resonant circuit that resonates at a wireless power receiving circuit resonant frequency. The wireless power transmitting device has coils. The coils are supplied with a drive signal in bursts to detect external objects. Measurement circuitry includes an oscillator for supplying the drive signals and a peak detector and analog-to-digital converter for gathering measurements on the coils to which the drive signals have been supplied. Rate-based-filtering is applied to output signals from the analog-to-digital converter to distinguish between temperature drift effects and object placement effects. The frequency of the drive signals is slightly greater than the wireless power receiving circuit resonant frequency.

This application is a continuation-in-part of U.S. application Ser. No.15/804,145, filed Nov. 6, 2017, which is hereby incorporated byreference herein in its entirety, and which claims the benefit ofprovisional patent application No. 62/453,850, filed on Feb. 2, 2017,and provisional patent application No. 62/526,285, filed on Jun. 28,2017, which are hereby incorporated by reference herein in theirentireties.

FIELD

This relates generally to wireless systems, and, more particularly, tosystems in which devices are wirelessly charged.

BACKGROUND

In a wireless charging system, a wireless power transmitting device suchas a device with a charging surface wirelessly transmits power to aportable electronic device. The portable electronic device receives thewirelessly transmitted power and uses this power to charge an internalbattery or to power the device. In some situations, foreign objects maybe accidentally place on a charging surface. This can pose challengeswhen performing wireless power transmission operations.

SUMMARY

A wireless power transmitting device transmits wireless power signals toa wireless power receiving device. The wireless power transmittingdevice has an inverter that supplies signals to an output circuit thatincludes a wireless power transmitting coil. The wireless powertransmitting coil may be part of an array of wireless power transmittingcoils that cover a wireless charging surface associated with thewireless power transmitting device.

Signal measurement circuitry is coupled to the output circuit to helpdetermine whether the wireless power receiving device is present andready to accept transmission of wireless power. The measurementcircuitry includes a measurement circuit that is coupled to the outputcircuit and that measures signals while oscillator circuitry suppliesthe output circuit with signals at a probe frequency. Using measurementsfrom this measurement circuitry at one or more probe frequencies, thewireless power transmitting device determines whether an external objectis present on the coils.

Impulse response circuitry in the measurement circuitry is coupled tothe output circuit and used to measure the response of the outputcircuit to an impulse signal supplied by an inverter in the wirelesspower transmitting device. The impulse response circuitry is used tomake inductance and Q factor measurements.

During operation, information from the impulse response circuitry andmeasurements at the probe frequency can be used in determining whether awireless receiving device is present over particular coils in wirelesscharging surface and can therefore be used in adjusting wireless powertransmission with the wireless power transmitting device.

The measurement circuitry also includes a measurement circuit that iscoupled to the output circuit and that measures signals while theoscillator circuitry sweeps an alternating-current output signal betweena first frequency and a second frequency. Measurements resulting fromfrequency-sweeping operations are used to detect sensitive devices suchas radio-frequency identification devices. If sensitive devices aredetected, potentially damaging wireless power transmission operationscan be avoided.

Switching circuitry is used to dynamically switch selected coils fromthe coil array that overlaps the charging surface into the outputcircuit, so that appropriate coils in the coil array can be probed forthe presence of external objects and sensitive devices such asradio-frequency identification devices.

The wireless power receiving device has a wireless power receiving coilin a resonant circuit that resonates at a wireless power receivingcircuit resonant frequency. The coils of the wireless power transmittingdevice are supplied with a drive signal in bursts. During each burst ofdrive signals, external objects can be detected. Between bursts, thedrive signals are not applied and measurements on the coils are notmade. This helps conserve power.

The measurement circuitry includes an oscillator for supplying the drivesignals and a peak detector and analog-to-digital converter forgathering measurements on the coils to which the drive signals have beensupplied. Rate-based-filtering is applied to output signals from theanalog-to-digital converter to distinguish between temperature drifteffects and object placement effects. The frequency of the drive signalsis slightly greater than the wireless power receiving circuit resonantfrequency to enhance signal measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative wireless chargingsystem in accordance with some embodiments.

FIG. 2 is a top view of an illustrative wireless power transmittingdevice with an array of coils that forms a wireless charging surface inaccordance with an embodiment.

FIG. 3 is a circuit diagram of illustrative wireless power transmittingcircuitry with output circuit signal measurement circuitry in a wirelesspower transmitting device in accordance with an embodiment.

FIG. 4 is a graph showing the response of various illustrative objectson the surface of a wireless power transmitting device in accordancewith an embodiment.

FIG. 5 is a graph of an illustrative impulse response of the type thatmay be used to characterize objects on a wireless power transmittingdevice in accordance with an embodiment.

FIG. 6 is a graph showing output signal traces of the type that may beassociated with placing a sensitive object such as a radio-frequencyidentification device on the surface of a wireless power transmittingdevice in accordance with an embodiment.

FIG. 7 is a cross-sectional side view of an illustrative portableelectronic device that has a wireless power receiving coil and anancillary coil that forms a resonant circuit in accordance with anembodiment.

FIG. 8 is a graph in which output circuit signal measurements as afunction of frequency have been plotted for multiple types ofillustrative objects on the surface of a wireless power transmittingdevice in accordance with an embodiment.

FIG. 9 is a flow chart of illustrative operations involved in operatinga wireless power transfer system such as the wireless charging system ofFIG. 1 in accordance with an embodiment.

FIG. 10 is a graph showing signals associated with performing impulseresponse measurements in a noisy wireless charging environment inaccordance with an embodiment.

FIG. 11 is a flow chart of illustrative operations associated withmaking impulse response measurements in accordance with embodiments.

FIG. 12 is a schematic diagram of an illustrative wireless chargingsystem having measurement circuitry in accordance with an embodiment.

FIG. 13 is a graph showing how a drive frequency for a measurementcircuit may be selected based on the resonant frequency of a wirelesspower receiving circuit in a wireless power receiving device in thewireless charging system in accordance with an embodiment.

FIG. 14 is a graph showing how drive signals may be applied to a coilarray in bursts separated by time periods with no drive signals toconserve power in accordance with an embodiment.

FIG. 15 is a graph showing how analog-to-digital converter outputsignals in a measurement circuit may be filtered using a windowalgorithm to help discriminate between temperature drive effects andexternal object movement effects in accordance with an embodiment.

DETAILED DESCRIPTION

A wireless power system has a wireless power transmitting device thattransmits power wirelessly to a wireless power receiving device. Thewireless power transmitting device is a device such as a wirelesscharging mat, wireless charging puck, wireless charging stand, wirelesscharging table, or other wireless power transmitting equipment. Thewireless power transmitting device has one or more coils that are usedin transmitting wireless power to one or more wireless power receivingcoils in the wireless power receiving device. The wireless powerreceiving device is a device such as a cellular telephone, watch, mediaplayer, tablet computer, pair of earbuds, remote control, laptopcomputer, other portable electronic device, or other wireless powerreceiving equipment.

During operation, the wireless power transmitting device suppliesalternating-current signals to one or more wireless power transmittingcoils. This causes the coils to transmit alternating-currentelectromagnetic signals (sometimes referred to as wireless powersignals) to one or more corresponding coils in the wireless powerreceiving device. Rectifier circuitry in the wireless power receivingdevice converts received wireless power signals into direct-current (DC)power for powering the wireless power receiving device.

An illustrative wireless power system (wireless charging system) isshown in FIG. 1. As shown in FIG. 1, wireless power system 8 includeswireless power transmitting device 12 and one or more wireless powerreceiving devices such as wireless power receiving device 10. Device 12may be a stand-alone device such as a wireless charging mat, may bebuilt into furniture, or may be other wireless charging equipment.Device 10 is a portable electronic device such as a wristwatch, acellular telephone, a tablet computer, or other electronic equipment.Illustrative configurations in which device 12 is a mat or otherequipment that forms a wireless charging surface and in which device 10is a portable electronic device that rests on the wireless chargingsurface during wireless power transfer operations may sometimes bedescribed herein as an example.

During operation of system 8, a user places one or more devices 10 onthe charging surface of device 12. Power transmitting device 12 iscoupled to a source of alternating-current voltage such asalternating-current power source 50 (e.g., a wall outlet that suppliesline power or other source of mains electricity), has a battery such asbattery 38 for supplying power, and/or is coupled to another source ofpower. A power converter such as AC-DC power converter 40 can convertpower from a mains power source or other AC power source into DC powerthat is used to power control circuitry 42 and other circuitry in device12. During operation, control circuitry 42 uses wireless powertransmitting circuitry 34 and one or more coils 36 coupled to circuitry34 to transmit alternating-current electromagnetic signals 48 to device10 and thereby convey wireless power to wireless power receivingcircuitry 46 of device 10.

Power transmitting circuitry 34 has switching circuitry (e.g.,transistors in an inverter circuit) that are turned on and off based oncontrol signals provided by control circuitry 42 to create AC currentsignals through appropriate coils 36. As the AC currents pass through acoil 36 that is being driven by the inverter circuit,alternating-current electromagnetic fields (wireless power signals 48)are produced that are received by one or more corresponding coils 14coupled to wireless power receiving circuitry 46 in receiving device 10.When the alternating-current electromagnetic fields are received by coil14, corresponding alternating-current currents and voltages are inducedin coil 14. Rectifier circuitry in circuitry 46 converts received ACsignals (received alternating-current currents and voltages associatedwith wireless power signals) from one or more coils 14 into DC voltagesignals for powering device 10. The DC voltages are used in poweringcomponents in device 10 such as display 52, touch sensor components andother sensors 54 (e.g., accelerometers, force sensors, temperaturesensors, light sensors, pressure sensors, gas sensors, moisture sensors,magnetic sensors, etc.), wireless communications circuits 56 forcommunicating wirelessly with control circuitry 42 of device 12 and/orother equipment, audio components, and other components (e.g.,input-output devices 22 and/or control circuitry 20) and are used incharging an internal battery in device 10 such as battery 18.

Devices 12 and 10 include control circuitry 42 and 20. Control circuitry42 and 20 includes storage and processing circuitry such asmicroprocessors, power management units, baseband processors, digitalsignal processors, microcontrollers, and/or application-specificintegrated circuits with processing circuits. Control circuitry 42 and20 is configured to execute instructions for implementing desiredcontrol and communications features in system 8. For example, controlcircuitry 42 and/or 20 may be used in determining power transmissionlevels, processing sensor data, processing user input, processing otherinformation such as information on wireless coupling efficiency fromtransmitting circuitry 34, processing information from receivingcircuitry 46, using information from circuitry 34 and/or 46 such assignal measurements on output circuitry in circuitry 34 and otherinformation from circuitry 34 and/or 46 to determine when to start andstop wireless charging operations, adjusting charging parameters such ascharging frequencies, coil assignments in a multi-coil array, andwireless power transmission levels, and performing other controlfunctions. Control circuitry 42 and/or 20 may be configured to performthese operations using hardware (e.g., dedicated hardware or circuitry)and/or software (e.g., code that runs on the hardware of system 8).Software code for performing these operations is stored onnon-transitory computer readable storage media (e.g., tangible computerreadable storage media). The software code may sometimes be referred toas software, data, program instructions, instructions, or code. Thenon-transitory computer readable storage media may include non-volatilememory such as non-volatile random-access memory (NVRAM), one or morehard drives (e.g., magnetic drives or solid state drives), one or moreremovable flash drives or other removable media, other computer readablemedia, or combinations of these computer readable media or otherstorage. Software stored on the non-transitory computer readable storagemedia may be executed on the processing circuitry of control circuitry42 and/or 20. The processing circuitry may include application-specificintegrated circuits with processing circuitry, one or moremicroprocessors, or other processing circuitry.

Device 12 and/or device 10 may communicate wirelessly. Devices 10 and 12may, for example, have wireless transceiver circuitry in controlcircuitry 42 and 20 (and/or wireless communications circuitry such ascircuitry 56 of FIG. 1) that allows wireless transmission of signalsbetween devices 10 and 12 (e.g., using antennas that are separate fromcoils 36 and 14 to transmit and receive unidirectional or bidirectionalwireless signals, using coils 36 and 14 to transmit and receiveunidirectional or bidirectional wireless signals, etc.).

With one illustrative configuration, wireless transmitting device 12 isa wireless charging mat or other wireless power transmitting equipmentthat has an array of coils 36 that supply wireless power over a wirelesscharging surface. This type of arrangement is shown in FIG. 2. In theexample of FIG. 2, device 12 has an array of coils 36 that lie in theX-Y plane. Coils 36 of device 12 are covered by a planar dielectricstructure such as a plastic member or other structure forming chargingsurface 60. The lateral dimensions (X and Y dimensions) of the array ofcoils 36 in device 36 may be 1-1000 cm, 5-50 cm, more than 5 cm, morethan 20 cm, less than 200 cm, less than 75 cm, or other suitable size.Coils 36 may overlap or may be arranged in a non-overlappingconfiguration. Coils 36 can be placed in a rectangular array having rowsand columns and/or may be tiled using a hexagonal tile pattern or otherpattern.

During operation, a user places one or more devices 10 on chargingsurface 60. Foreign objects such as coils, paper clips, scraps of metalfoil, and/or other foreign conductive objects may be accidentally placedon surface 60. System 8 automatically detects whether conductive objectslocated on surface 60 correspond to devices 10 or incompatible foreignobjects and takes suitable action. With one illustrative arrangement,system 8 checks whether objects located on surface 60 include sensitiveitems such as radio-frequency identification (RFID) devices or otherpotentially sensitive electronic equipment that could be potentiallydamaged upon exposure to large fields from coils 36 before system 8allows wireless power to be transmitted to those objects.

As shown in the example of FIG. 2, external objects such as externalobject 62 and object 64 may overlap one or more coils 36. In somesituations, objects 62 and 64 will be portable electronic devices 10. Inother situations, one or more of objects 62 and 64 will be incompatibleexternal objects (e.g., conductive foreign objects such as metalliccoins, sensitive devices such as RFID devices, etc.). Situations mayalso arise in which incompatible external objects and portableelectronic devices overlap the same coil or coils 36.

Illustrative wireless power transmitting circuitry 34 that includescircuitry to detect and characterize external objects on surface 60 isshown in FIG. 3. As shown in FIG. 2, circuitry 34 may include aninverter such as inverter 72 or other drive circuit that produceswireless power signals that are transmitted through an output circuitthat includes one or more coils 36. A single coil 36 is shown in theexample of FIG. 2. In general, device 12 may have any suitable number ofcoils 36 (1-100, more than 5, more than 10, fewer than 40, fewer than30, 5-25, etc.). Switching circuitry MX (sometimes referred to asmultiplexer circuitry) that is controlled by control circuitry 42 can belocated before and/or after each coil 36 and/or before and/or after theother components of output circuit 71 and can be used to switch desiredsets of one or more coils 36 (desired output circuits 71) into or out ofuse. For example, if it is determined that object 62 of FIG. 2 is awireless power receiving device 10 and object 64 is an incompatibleforeign object such as a coin, the coils overlapping object 62 may beactivated during wireless power transmission operations and the coilsunder object 64 may be deactivated so that these coils do not transmitwireless power. Other coils 36 (e.g., coils not overlapped by object 64in this example) can also be turned off during wireless powertransmission operations, if desired.

With continued reference to FIG. 3, during wireless power transmissionoperations, transistors 74 of inverter 72 are driven by AC controlsignals from control circuitry 42. Control circuitry 42 may also usetransistors 74 of inverter 72 to apply square wave pulses or otherimpulses to coil 36 (e.g., during impulse response measurements). Coil36 (e.g., a coil that has been selected using multiplexing circuitry MX)has an inductance L. Capacitor 96 has a capacitance C1 that is coupledin series with inductance L in output circuit 90. When supplied withalternating-current drive signals from inverter 72 while switch(transistor) TP is closed, the output circuit formed from coil 36 andcapacitor 96 produces alternating-current electromagnetic fields thatare received by one or more coils 14 in device 10. The inductance L ofeach coil 36 is influenced by magnetic coupling with external objects,so measurements of inductance L for one or more of coils 36 in device 12at various frequencies can reveal information on objects on chargingsurface 60.

To conserve power, device 12 may be operated in a standby mode whileawaiting use to supply wireless power to devices 10. The signalmeasurement circuitry of FIG. 3 (sometimes referred to as output circuitsignal measurement circuitry, external or foreign object detectioncircuitry, etc.) monitors for the presence of external objects duringstandby. The power consumption of the measurement circuitry intransmitter circuitry 34 during standby operations may be less than 50mW, less than 200 mW, more than 1 mW, or other suitable value.

In standby mode, device 12 periodically scans coils 36 (e.g., device 12scans each of coils 36) for the presence of external objects (e.g.,devices 10, foreign objects such as coins, etc.). To probe a selectedcoil for changes in inductance L due to external objects, a probe signalis driven onto node N1 with oscillator circuitry 84 while controlcircuitry 42 turns off inverter 72 (e.g., transistors 74 are not used todrive signals onto node N2). Control circuitry 42 may, for example, useoscillator circuitry 84 (e.g., one or more voltage controlledoscillators, one or more other adjustable oscillators, and/or otheroscillatory circuitry) to produce an alternating-current probe signal(e.g., a sine wave, square wave, etc.) at a probe frequency fr (e.g., 4MHz or other suitable frequency such as a frequency of at least 500 kHz,at least 1 MHz, at least 2 MHz, less than 10 MHz, between 1 MHz and 10MHz, or other suitable frequency). The probe frequency fr that is usedduring standby mode is a frequency that differs from RFID frequenciessuch as 13.56 MHz and that differs from the normal alternating-currentfrequency supplied to output circuit 71 by inverter 72 during wirelesscharging operations, which may be, for example, 100-500 kHz, more than50 kHz, more than 100 kHz, more than 200 kHz, less than 450 kHz, lessthan 400 kHz, less than 300 kHz, or other suitable wireless poweralternating-current drive frequency.

The signal at frequency fr is applied to node N1 via capacitor 86 andcoupled to coil 36 via capacitor 96 while inverter 72 is held in an offstate by control circuitry 42. Control circuitry 42 controls multiplexerMX to select the coil to which the signal at frequency fr is applied(e.g., coil 36 of FIG. 3) from the array of coils 36 of device 12 shownin FIG. 2. Capacitance C1 may have a value of 150 μF, more than 10 μF,less than 1000 μF, or other suitable value. Transistor TP may have aparasitic capacitance Cp (e.g., a capacitance of 80 pF, more than 10 pF,less than 800 pF, or other suitable value) when open. For standbyoperations, control circuitry 42 opens transistor TP so that so thatprobe signals are routed through coil 36. When transistor TP is open,parasitic capacitance Cp is coupled in series with capacitance C1. Thiseffectively removes capacitance C1 from the series circuit formed withinductance L, as the capacitance of capacitance C1 (which is in themicrofarad range) and Cp (which is in the picofarad range) in serieswill be approximately Cp.

With TP open, output circuit 71 (coil 36 in series with C1 and Cp) willbe characterized by a resonance at frequency fres of equation 1.fres=1/(2π(LCp)^(1/2))  (1)

The expected measured signal at node N1 (output voltage OUT(N1)) as afunction of applied signal frequency f in the absence of externalobjects on coil 36 is given by curve 102 of FIG. 4. In the presence ofan electronic device such as device 10 that contains one or more coils14 overlapping coil 36, curve 102 may shift to lower frequencies asshown by curve 100. In the presence of a coin or other incompatibleforeign object overlapping coil 36, curve 102 may shift to higherfrequencies as shown by curve 104. Changes in load can be detected bymonitoring the value of OUT(N1) using measurement circuit 78 of FIG. 3at one or more probe frequencies. For example, oscillator circuitry 84may be used to apply a probe signal to node N1 at a frequency fr thathas been chosen to match resonant frequency fres of equation 1. Ifdesired, multiple probe signals may be applied to output circuit 72while using measurement circuitry to evaluate the resulting signal onnode N1. For example, the direction of change in curve 102 (shiftinghigher or lower) can be detected by taking multiple measurements ofOUT(N1) at two or more frequencies near frequency fr of FIG. 4).

To make measurements of OUT(N1), measurement circuit 78 includes peakdetector 80 and analog-to-digital converter 82. Circuit 78 measures thesignal at node N1 and supplies a corresponding digital version of thissignal to control circuitry 42. In the presence of an object overlappingcoil 36 (whether from device 10, a sensitive RFID device, or a coin orother incompatible foreign object), signal OUT(N1) will drop. Forexample, the signal on node N1 may drop from a value of P1 (e.g., a peakvalue associated with curve 102) when coil 36 is unloaded to a value ofP2 (a reduced value associated with shifted curve 100) when coil 36 isloaded due to the presence of an external object.

During standby operations, control circuitry 42 can scan through coils36 by using multiplexer circuitry MX or other switching circuitry incircuitry 34. In some embodiments, this sequentially couples each ofcoils 36 to node N1 while circuitry 78 measures OUT(N1) for eachselected coil 36. If no changes in OUT(N1) are detected, controlcircuitry 42 can conclude that no objects are present on device 12(e.g., no objects are resting on charging surface 60). If a change inOUT(N1) is detected, control circuitry 42 performs additional operationsto confirm that device 10 is present rather than an incompatible foreignobject such as a coin.

With one illustrative approach, control circuitry 42 uses impulseresponse measurement circuitry 76 (sometimes referred to as inductancemeasurement circuitry and/or Q factor measurement circuitry) to performlow-frequency measurements of inductance L and quality factor Q inresponse to detection of a load on one or more coils 36 during standby.During impulse response measurements, control circuitry 42 directsinverter 72 to supply one or more excitation pulses (impulses) to coil36 while turning on transistor TP, so that L and C1 in output circuit 71form a resonant circuit. The impulses may be, for example, square wavepulses of 1 μs in duration. Longer or shorter pulses may be applied, ifdesired. The resonant circuit may resonate at a frequency near to thenormal wireless charging frequency of coil 36 (e.g., about 320 kHz,100-500 kHz, more than 50 kHz, more than 100 kHz, more than 200 kHz,less than 450 kHz, less than 400 kHz, less than 300 kHz, or othersuitable wireless charging frequency).

The impulse response (signal OUT(N1)) of circuit 71 to the appliedpulse(s) is as shown in FIG. 5. The frequency of the impulse responsesignal of FIG. 5 is proportional to 1/sqrt(LC), so L can be obtainedfrom the known value of C1 and the measured frequency of the impulseresponse signal. Q may be derived from L and the measured decay of theimpulse response signal. As shown in FIG. 5, if signal OUT(N1) decaysslowly, Q is high (e.g., HQ) and if signal OUT(N1) decays more rapidly,Q is low (e.g., SQ). Measurement of the decay envelope of OUT(N1) andfrequency of OUT(N1) of the impulse response signal of FIG. 5 withcircuitry 76 will therefore allow control circuitry 42 to determine Qand L.

If the measured value of L for a given coil matches the normal L valueexpected for each of coils 36 in the array of coils 36 overlappingsurface 60 (e.g., when the measured L value is not influenced by thepresence device 10 or other external object on surface 60), controlcircuitry 42 can conclude that no external object suitable for wirelesscharging is present. If a given measured value of L is larger than thatexpected for an unloaded coil, control circuitry 42 can conclude that anexternal object is present that is suitable for wireless charging andcan perform additional measurement operations. For example, controlcircuitry 42 can perform a swept-frequency measurement (sometimesreferred to as an RFID checking measurement) on node N1 to check whethera sensitive device such as an RFID device is present on surface 60.

The measurements made by circuitry 76 are performed on one or more ofcoils 36 (e.g., these measurements may be performed on each of coils 36in the array of coils in device 12). Circuitry 42 uses these impulseresponse measurements to identify spatial patterns in measured L values(and/or Q factor values) across surface 60. Analysis of a pattern ofmeasured inductance (L) change can help determine whether a known typeof device 10 is present on coils 36. Analysis of the spatial patterns ofmeasured inductance L (and, if desired, Q factor, which has an inverserelationship with respect to L), as a function of coil position in theX-Y plane of surface 60 may be used in determining when to transitwireless power from device 12 to device 10. If, for example, the valueof L for each of coils 36 is unchanged from its nominal state, circuitry42 can conclude that no external device suitable for wireless chargingis present. If the value of L for a given one of coils 36 is elevated orother suitable pattern of measured L values is detected, circuitry 42can conclude that an external device that is suitable for wirelesscharging is present on that coil and can prepare to transmit wirelesspower using that coil.

Before transmitting wireless power, it may be desirable to check whethera sensitive device such as an RFID device is present on surface 60.Sensitive devices can potentially be harmed by excessive wireless powerlevels, so checking for sensitive devices helps avoid damage tosensitive devices during subsequent wireless power transfer operations.In some scenarios, both portable device 10 and a sensitive device may bepresent over the same coil 36 in the array of coils 36 in device 12. Asensitive device may, as an example, be present under a cellulartelephone, watch, or other portable device 10 that includes a wirelesspower receiving coil 14. Even though the presence of the portable device10 can be detected by making inductance measurements with coils 36, itis desirable to check whether a sensitive device is also present so asto avoid damaging the sensitive device by exposure to wireless powertransmissions.

Radio-frequency identification (RFID) devices typically have RFID coilcircuits that resonate at relatively high frequencies such as afrequency of 13.56 MHz. In some embodiments, to determine if an RFID ispresent on surface 60, RFID checking measurements are performed bymeasuring signal OUT(N1) on node N1 using measurement circuit 94 (FIG.3). During these checking measurements, control circuitry 42 directsoscillator circuitry 84 to sweep the frequency of the signal supplied tonode N1 between a first frequency f1 and a second frequency f2 coveringthe expected resonant frequencies of popular RFID coils. Transistor TPmay remain open so that current from oscillator circuitry 84 flowsthrough each coil 36 that has been selected during measurementoperations. The value of f1 may be, for example, 10 MHz, more than 5MHz, less than 11 MHz, less than 12 MHz, less than 15 MHz, or othersuitable value. The value of f2 may be 30 MHz, more than 14 MHz, morethan 15 MHz, more than 20 MHz, less than 45 MHz, or other suitablevalue.

As shown in FIG. 3, swept-frequency measurement circuit 94 includes apeak detector such as peak detector 88 that measures the voltage on nodeN1, band pass filter 90, and analog-to-digital converter circuitry 92.Analog-to-digital converter circuitry 92 supplies a digital version ofits input to control circuitry 42.

When no RFID device is present on charging surface 60 of device 12, peakdetector 88 will detect a signal such as the signal of curve 108 in FIG.6. When an RFID device overlaps charging surface 60, signal OUT(N1)(see, e.g., curve 110) will exhibit a resonance signal such as signal112 in as frequency f is swept between f1 and f2. Resonance signal 112may, for example, correspond to a resonance frequency such as an RFIDresonant frequency of 13.56 MHz.

Frequency f is swept between f1 and f2 at a predetermined speed. Forexample, control circuitry 42 may sweep frequency from f1 to f2 in aninterval of 2 ms, at least 1 ms, less than 3 ms, or other suitable timeperiod. The pass frequency of band pass filter 90 is selected so thatresonance signal 112 will pass through band pass filter 90 as band passfiltered signal 112′ of band pass output curve 114 when frequency f ischanged between f1 and f2 at the predetermined speed (e.g., when thefull sweep range is covered in an interval of 2 ms, etc.). The use ofband pass filter 90 helps remove non-resonant signal fluctuations fromcurve 110 (e.g., signal tilt and slowly varying increases and/ordecreases of the type shown by illustrative curve 110 of FIG. 6). Theresulting band-pass-filtered signal (curve 114 and filtered signalresonance 112′) can be processed by control circuitry 42 to confirm thatan RFID resonance at a particular frequency has been detected. Controlcircuitry 42 can then take appropriate action. For example, if no RFIDsignature is detected, control circuitry 42 can conclude that thedetected external object on surface 60 is likely a portable device(device 10 with coil 14) without any intervening (overlapping) sensitiveRFID device. If an RFID signature (e.g., resonant signal 112′ at an RFIDfrequency such as 13.56 MHz) is detected, control circuitry 42 canreduce the level of wireless power transmitted by coils 36 or canprevent wireless power from being transmitted by coils 36 (or at leastthe coils that are overlapped by the sensitive RFID device) so as tomitigate damage to the RFID device. Optionally, control circuit 42 canissue an alert to a user.

In some arrangements, it may be desirable to avoid sensitive frequenciesduring the frequency sweep operations of FIG. 6. For example, it may bedesirable to skip a narrow band of frequencies centered on anunpermitted frequency fnp such as band 113. Unpermitted frequency fnpmay be, as an example, a frequency of 13.56 MHz. Band 113 may coverfrequencies within +/−20 kHz of 13.56 MHz (as an example). Skipping band113 during the frequency sweep from f1 to f2 may ensure regulatorycompliance in jurisdictions in which use of the frequencies of band 113is restricted. To facilitate skipping of band 113, oscillator 84 may beimplemented using a circuit that permits rapid skipping of undesiredfrequencies during frequency sweeping such as a direct digital sine wavegenerator. Other types of oscillator may be used, if desired.

FIG. 7 is a cross-sectional side view of device 10 in an illustrativeconfiguration in which device 10 has a power receiving coil (coil 14)located in the lower portion of device housing 116. Device may also haveone or more additional coils such as coil PR. Each optional coil PR mayform part of a corresponding resonant circuit (e.g., a passive resonantcircuit with a known frequency resonance at a frequency between 10 MHzand 30 MHz or other suitable frequency). The measurement circuitry ofdevice 12 can detect the presence and location of coils such as coil PRwhen scanning frequency f for each coil 36 as described in connectionwith FIG. 6. The incorporation of known passive resonators into device10 may help allow the location, orientation, and type of device 10 to beaccurately identified by device 12.

Different devices may also have different known frequency resonanceswhen placed on surface 60. Consider, for example, the scenario of FIG.8. In the absence of an external object, coil 36 may exhibit a frequencyresponse of the type shown by curve FS. When a first type of device 10(e.g., a cellular telephone) is placed on surface 60, curve FS may shiftto curve D1. When a second type of device 10 (e.g., a watch) is placedon surface 60, curve FS may shift to curve D2. By measuring OUT(N1) bysweeping across a predetermined frequency range (e.g., from a lowfrequency of 1 kHz, 10 kHz, more than 100 kHz, more than 1 MHz, morethan 10 MHz, less than 100 MHz, less than 10 MHz, less than 1 MHz, orother suitable low frequency to a high frequency of 10 kHz, more than100 kHz, more than 1 MHz, more than 10 MHz, more than 10 MHz, less than1 GHz, less than 100 MHz, less than 10 MHz, or other suitable highfrequency), device 12 can determine what type of power receiving device10 is present and can use this information to take appropriate action(e.g., by supplying wireless power to that device withdevice-appropriate settings, etc.). If desired, circuitry 42 may alsodiscriminate between curves such as curves FS, D1, and D2 using smallersets of measurements (e.g., a set of 2-10 data points, more than 2 datapoints, fewer than 5 data points, etc.).

FIG. 9 is a flow chart of illustrative operations involved in usingsystem 8. During the operations of block 120, system 8 performs standbymeasurements. For example, device 12 may use circuitry such as circuit78 of FIG. 3 to monitor one or more of coils 36 (e.g., each coil 36 inthe array of coils 36 in device 12) for the presence of an externalobject such as one of devices 10 which is potentially compatible forwireless power transfer or an incompatible object such as a coin orbadge. A single measurement at frequency fr may be made to determinewhether OUT(N1) is lower than expected for any coils 36 or, if desired,multiple measurements at different frequencies near fr may be made(e.g., to determine which direction the coil resonance has shifted dueto an external object and thereby help determine whether the object isan electronic device or is a coin or other incompatible foreign object).The standby operations of block 120 consume a low amount of power (e.g.,50 mW or less, 100 mW or less, more than 1 mW, or other suitableamount).

In response to detection of an external object with control circuitry 42during the operations of block 120, control circuitry 42 performsadditional detection operations such as low-frequency impulse responsemeasurements (block 122). During the operations of block 122, controlcircuitry 42 may, for example, use inverter 72 or other resonant circuitdrive circuitry to apply a stimulus (e.g., a square wave or other signalimpulse) to the circuit formed from one or more of coils 36 (e.g., toeach coil 36 in the array of coils 36 in device 12, a subset of thesecoils such as those for which foreign object presence has been detectedduring the operations of block 120, and/or other suitable sets of one ormore of coils 36), thereby causing that circuit (and that coil 36) toresonate while using a measurement circuit such as impulse responsemeasurement circuitry 76 of FIG. 3 to measure the response of theresonant circuit. As described in connection with FIG. 5, thecharacteristics of the resulting circuit resonance may then be measuredand analyzed. For example, control circuitry 42 may use information onthe measured resonant frequency to measure inductance and may useinformation on the decay of the signal resonance to determine resistanceR and Q factor. If desired, the measurements of blocks 120 and/or 122can be mapped in dimensions X and Y across surface 60 to help identifydevices 10 and foreign objects.

If the operations of block 122 reveal that no foreign object is presentand that an electronic device 10 is present, additional checkingoperations may be performed during block 124. In particular, frequencysweep measurements with circuitry such as oscillator circuitry 84 andswept-frequency measurement circuit 94 of FIG. 3 may be performed tocheck for the presence of a sensitive RFID device, as described inconnection with FIG. 6.

Appropriate action are taken during the operations of block 126 based onthe results of measurements such as the measurements of blocks 120, 122,and/or 124. If, as an example, a sensitive RFID device is detectedduring the operations of block 124 or if a foreign object is detected,wireless charging operations with all of coils 36 or an appropriatesubset of coils 36 can be blocked. In response to detection of anelectronic device 10 having a known characteristic L response (and/or Qresponse) and in response to determining that no RFID device is presentafter checking one or more of coils 36, as appropriate, with circuit 94(e.g., the coils 36 for which L and/or Q measurements and/or othermeasurements indicate may be overlapped by an object or all of coils36), control circuitry 42 can use wireless power transmitting circuitry34 to transmit wireless power to wireless power receiving circuitry 46.

In some operating environments, signal measurement accuracy can beadversely affected by noise. For example, in arrangements in whichmultiple power receiving devices are located on a common wirelesscharging mat, the process of transmitting wireless power to one of thedevices using coils in the mat that are overlapped by that device maycreate noise when making measurements such as impulse responsemeasurements on another device that overlaps different coils in the mat.With one illustrative arrangement, potential interference can be avoidedby stopping the charging of a first device for a sufficient amount oftime to allow measurements such as impulse response measurements to bemade on a second device in the absence of noise. With anotherillustrative arrangement, noise can be removed from measurements such asimpulse response measurements.

FIG. 10 is a diagram showing how the measured signal on node N1 (orother suitable node) such as signal OUT may contain noise whenattempting to make measurements on a coil overlapped by a wireless powerreceiving device. As shown by curve 150, signal OUT can initially bemeasured during time period T1 by impulse response measurement circuitry76, in the absence of any applied impulse by impulse responsemeasurement circuitry 76. There may be noise present in the signal OUTduring time period T1 due to the charging of one or more additionalwireless power receiving devices elsewhere on device 12 (e.g.,overlapping other coils besides the coil/coils 36 overlapped by thewireless power receiving device). During time period T2, impulseresponse measurement circuitry 76 applies an impulse to coil 36 andmeasures the resulting ringing and exponential decay of the resultingsignal OUT. Noise is present in this measured signal due to the wirelesspower transmission to one or more other devices on device 12. Asdescribed in connection with FIG. 5, the frequency and decay rate of theimpulse response signal can reveal information such as the value of coilinductance L. To enhance measurement accuracy, the noise measured duringperiod T1 can be removed from the signal measured during period T2before processing the signal measured during period T2 to producemeasurement results such as inductance L.

A flow chart of illustrative operations involved in measuring inductanceL in a potentially noisy environment such as a charging environment inwhich multiple devices 10 are located on a common wireless powertransmitting device 12 is shown in FIG. 11. As shown in FIG. 11, powertransmission from device 12 to a first device 10 may be initiated atstep 152.

With one illustrative approach, power transmission to the first deviceis momentarily suspended to permit measurement of L for a coil(s)overlapped with a second device. This approach is illustrated by theoperations of blocks 154, 156, and 158. During the operations of block154, device 12 stops power transmission to the first device. During theoperations of block 156, impulse response measurement circuitry 76 isused to make impulse response measurements and thereby obtain L for thecoil overlapped by the second device (e.g., while the first device isnot receiving power). During the operations of block 158, powertransmission from device 12 to the first device 10 is resumed. Power canalso be wirelessly transmitted to the second device 10.

With another illustrative approach, which is illustrated in blocks 160,162, and 164, noise is removed from the measured signals withoutinterrupting power transmission to the first device. During theoperations of block 160, measurement circuitry 76 or other measurementcircuitry measures noise in signal OUT (see, e.g., period T1 of FIG. 10)for the coil(s) overlapped by the second device without interruptingpower transmission to the first device. During the operations of block162, the impulse response measurement circuitry 76 applies an impulsepulse to the coil(s) overlapped by the second device and measures signalOUT (see, e.g., period T2 of FIG. 10) while power continues to betransmitted to the first device using other coils in device 12 (therebyintroducing noise into the signal measured during time period T2).During the operations of block 164, impulse response measurementcircuitry 76 and/or control circuitry 42 removes the measured noise ofperiod T2 from the impulse response signal of period T2 (e.g., byrepeatedly subtracting the measured noise at various different trialphase values until satisfactory noise removal is achieved). If the noiseis satisfactorily removed in this way (e.g., if an exponential decay insignal OUT is obtained and the measured value of L and/or otherparameters is therefore obtained with satisfactory accuracy), impulseresponse measurement are complete and wireless power transmissionoperations may proceed. As shown by line 166, if noise is notsatisfactorily removed during the operations of block 164, processingmay loop back to block 160 to make additional noise and impulse responsemeasurements.

System 8 allows device 12 to forgo charging of device 10 if a foreignobject such as a radio-frequency identification (RFID) device isoverlapped by device 10 and is therefore interposed between device 10and one or more coils 36 in device 12. Because coils 36 can becontrolled independently, if a RFID device or other sensitive device isdetected on one portion of device 12 (e.g., overlapping a first set ofone or more coils) while device 10 is detected on another portion ofdevice 12 (e.g., overlapping a second set of one or more coils differentfrom the first set of coils), device 12 can transmit power wirelesslyusing only the second set of coils and not the first set of coils. Inthis way, power is not wirelessly transmitted to the RFID device but istransmitted only to wireless power receiving device 10.

FIG. 12 is a schematic diagram showing how wireless power transmittingcircuitry 34 may have circuitry of impedances Z1 and Z2 that form avoltage divider at node N1. Impedance Z1 may be formed from componentssuch as capacitor 86. Impedance Z2 may be formed by coil 36 andassociated circuit components (e.g., a parasitic resistance andcapacitance associated with coil 36). Multiplexer MX may include anarray of switches. When it is desired to switch as desired coil 36 intouse, its associated multiplexer switch can be closed. For example,control circuitry 42 can control multiplexer MX so that drive signalscan be applied to each of coils 36 when making measurements to detectwireless power receiving device 12 or other external object on coils 36.

When it is desired to perform object detection measurements on coils 36(impedance Z2), oscillator 84 drives an alternating-current drive signalsuch as a square wave signal onto node N1. The frequency of the drivesignal may be 1.1 MHz, at least 800 MHz, at least 1 MHz, less than 5MHz, less than 1.5 MHz, or other suitable frequency (as examples). Peakdetector 80 and analog-to-digital converter 82 of measurement circuit 78are used in measuring the voltage on node N1 to detect external objects.During operation of system 8 (e.g., when wireless power receiving device10 is being used to receive wireless power), wireless power receivingdevice 10 tunes its resonant circuit (e.g., a wireless power receivingdevice resonant circuit formed from a coil 14 and associatedcapacitance) so that the wireless power receiving device resonantexhibits a desired wireless power receiving circuit resonant frequencyfrx. The value of frx may be, for example, 1 MHz or other suitablefrequency (e.g., 0.9 to 1.1 MHz, 0.8 to 1.2 MHz, etc.). When device 10is present on device 12 (e.g., when coil 14 is placed on a coil 36 indevice 12), the impedance of the resonant circuit of device 10 isreflected to the transmitter's coil impedance. As a result, theinductance of coil 36 (Z2) exhibits a resonance at frequency frx asillustrated by curve 200 of FIG. 13.

For satisfactory detection of device 10 on each coil 36, the drivefrequency fdrive of oscillator 84 may be selected to be slightly largerthan receiver resonant frequency frx (e.g., fdrive may be 101% to 150%of frx, as an example). This ensures that the measured voltage of nodeN1 (e.g., the change in the voltage on node N1 due to the presence ofdevice 10) will be sufficient to be measured by measurement circuit 78.With one illustrative configuration, frx is 1 MHz and fdrive is between1 MHz and 1.5 MHz, 1.1-1.5 MHz, at least 1.05 MHz, at least 1.1 MHz, atleast 1.15 MHz, less than 2 MHz, less than 1.9 MHz, less than 1.8 MHz,less than 1.7 MHz, less than 1.6 MHz, less than 1.4 MHz, less than 1.3MHz or other suitable frequency that ensures that the voltage deflectionon node N1 will be sufficient in response to placement of device 10 (andits resonant circuit) on a given transmitter coil 36.

If desired, power can be conserved during measurements with measurementcircuit 78 by applying the drive signal from oscillator 84 in burstsseparated by periods of inactivity (no applied drive signals). As shownin FIG. 14, for example, a first burst (burst B1) may include a seriesof alternating-current drive signals applied to a first coil (coil C1),a second series of alternating-current drive signals applied to a secondcoil (coil C2), . . . up to an Nth series of signals for an Nth coil CN,followed by subsequent bursts of signals such as second burst (see,e.g., burst B2). Each coil scan may be about 0.5-2 ms in duration (as anexample). There may be 22 coils 36 or other suitable number of coils indevice 12 (e.g., the value of N can be 22). Within each burst ofmeasurement signals, alternating-current signals from oscillator 84 maybe applied to each coil (C1 . . . CN) in sequence. No drive signals areapplied to the coils during off period Toff between successive bursts.

As shown in FIG. 14, for example, following the measurements of burstB1, oscillator 84 does not supply any output signals for off period Toffto conserve power. Signal measurements are resumed after period Toff iscomplete. The value of Toff may be selected to be longer to reduce powerconsumption or shorter to reduce detection latency. With oneillustrative configuration, Toff has a value of 100 mS to 2 s. Ingeneral, Toff can have any suitable value (e.g., 200 ms, 225 ms, 250 ms,at least 10 ms, at least 50 ms, at least 100 ms, at least 200 ms, atleast 400 ms, at least 800 ms, less than 3 s, less than 1.5 s, less than900 ms, less than 500 ms, or less than 300 ms). If desired, measurementoperations with off period Toff may be used in configurations forwireless power transmitting device 12 in which only a single coil 36 ispresent. The use of burst mode operations in the context of a multi-coilsystem is illustrative.

Peak detector 80 may be implemented using diodes. For example, peakdetector 80 may be formed from a pair of diodes (e.g., a first diodecoupled to ground and a second diode coupled in series between node N1and analog-to-digital converter circuit 82). Diode forward voltages areaffected by temperature fluctuations. To reduce measurement inaccuraciesfrom temperature drift effects, the output from analog-to-digitalconverter 82 is time filtered (e.g., with a rate-based filterimplemented in software, firmware, and/or hardware using controlcircuitry 42). When a user places device 10 on device 12, the impedanceof coil 36 and therefore the output of analog-to-digital converter 82will fluctuate more rapidly than when the output of analog-to-digitalconverter 82 is affected by temperature drift. By filtering the outputof converter 82 (e.g., using an analog-to-digital converter speedthreshold of 5 analog-to-digital converter counts per 2 seconds or othersuitable threshold in applying rate-of-change-based filtering to theoutput of the analog-to-digital converter), control circuitry 42 can userate-of-change filtering to discriminate between a relatively fastchange in measured impedance due to placement of device 10 and arelatively slow change in measured impedance due to temperature drift.

With one illustrative configuration, a window algorithm is used toimplement a rate-of-change filter to filter the output ofanalog-to-digital converter 82 and thereby discriminate between deviceplacement events (which are to be detected) and temperature drifteffects (which are to be ignored). FIG. 15 is a graph in whichanalog-to-digital converter output (ADC) has been plotted as a functionof time (ADC scans). As shown in FIG. 15, control circuitry 42 canexamine ADC output values within a time window (e.g., time periodk−(k−K) of FIG. 15). With one illustrative configuration, all coils 36are scanned every 250 ms (or other suitable period). Each scan (one ADCvalue) is stored in a first-in-first-out (FIFO) buffer in memory incontrol circuitry 42, so that the buffer contains K scans for each coil.Control circuitry 42 then uses a window algorithm (filter process) toprocess the window data. In particular, the maximum of the ADC valuewithin the window minus the current ADC value (see, e.g., ΔADC of FIG.15) is compared to a predetermined object detection threshold value. Ifthe value of ΔADC minus the current ADC value exceeds a predeterminedthreshold amount, control circuitry 42 concludes that the change inoutput of converter 82 during the window is due to an object placementon device 12 (e.g., device 10 is being placed on device 12). If thevalue of ΔADC minus the current ADC value does not exceed thepredetermined threshold amount, control circuitry 42 concludes that thechange in output of converter 82 during the window is due to temperaturedrift and can be ignored. If desired, other techniques may be used bycontrol circuitry 42 to process analog-to-digital converter output todistinguish between temperature drift effects and object placementeffects. The use of a window algorithm to process analog-to-digitalconverter output is illustrative.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A wireless power transmitting device, comprising:a coil; wireless power transmitting circuitry coupled to the coil andconfigured to transmit wireless power signals to a wireless powerreceiving device with a receiving coil in a wireless power receivingcircuit that is configured to resonate at a wireless power receivingcircuit resonant frequency; control circuitry configured to controltransmission of the wireless power signals; an oscillator coupled to thecoil that is configured to apply a probe signal to the coil at a probefrequency; and an analog-to-digital converter configured to measuresignals at the probe frequency, wherein the control circuitry isconfigured to: in a standby mode, determine whether an external objectis present by directing the oscillator to supply the probe signal to thecoil in bursts separated by respective periods of time in which no probesignals are supplied to the coil by the oscillator, wherein the probefrequency of the probe signal is the same during each of the bursts,wherein the probe frequency is equal to 101% to 150% of the wirelesspower receiving circuit resonant frequency, wherein each burst has afirst duration greater than 0.5 milliseconds, and wherein each period oftime in which no probe signals are supplied to the coil by theoscillator has a second duration greater than 100 milliseconds.
 2. Thewireless power transmitting device of claim 1 wherein the probefrequency is between 1 MHz and 1.5 MHz.
 3. The wireless powertransmitting device of claim 1 wherein the wireless power receivingcircuit resonant frequency is 1 MHz and the probe frequency is at least1 MHz.
 4. The wireless power transmitting device of claim 1, wherein thecontrol circuitry is configured to: determine that the external objectis present in response to the analog-to-digital converter detecting avoltage drop at a node between the oscillator and the coil.
 5. Thewireless power transmitting device of claim 4, wherein the controlcircuitry is configured to: in response to determining that the externalobject is present, obtain measurements to determine whether the externalobject is the wireless power receiving device with the receiving coil ora foreign object.
 6. The wireless power transmitting device of claim 5,wherein the control circuitry is configured to: in response todetermining that the external object is the wireless power receivingdevice, transmit wireless power signals to the wireless power receivingdevice.
 7. The wireless power transmitting device of claim 5, whereinobtaining measurements to determine whether the external object is thewireless power receiving device with the receiving coil or the foreignobject comprises: using impulse response measurement circuitry,measuring decay of an impulse response signal that is produced inresponse to a pulse being applied to the coil.
 8. The wireless powertransmitting device of claim 1, wherein the first duration is between0.5 milliseconds and 2 milliseconds and wherein the second duration isgreater than 800 milliseconds.
 9. A wireless power transmitting device,comprising: a coil; wireless power transmitting circuitry coupled to thecoil and configured to transmit wireless power signals to a wirelesspower receiving device with a receiving coil in a wireless powerreceiving circuit that is configured to resonate at a wireless powerreceiving circuit resonant frequency; control circuitry configured tocontrol transmission of the wireless power signals; and measurementcircuitry coupled to the coil that is used by the control circuitry todetect external objects, wherein the measurement circuitry includes anoscillator configured to apply a probe signal at a probe frequency thatis larger than the wireless power receiving circuit resonant frequencyand includes an analog-to-digital converter configured to measuresignals at the probe frequency, wherein the probe frequency is equal to101% to 150% of the wireless power receiving circuit resonant frequency,and wherein the control circuitry is configured to process output fromthe analog-to-digital converter to distinguish between temperature drifteffects and object placement effects.
 10. A method of using a wirelesspower transmitting device having wireless power transmitting circuitrythat transmits wireless power signals to a wireless power receivingdevice using a coil, wherein the wireless power receiving device has awireless power receiving circuit with an associated wireless powerreceiving circuit resonant frequency, the method comprising: in astandby mode, monitoring the coil for presence of an external object byperiodically supplying, with an oscillator, a probe signal to the coilat a probe frequency equal to 101% to 150% of the wireless powerreceiving circuit resonant frequency; in response to detecting theexternal object, obtaining measurements to determine whether theexternal object is the wireless power receiving device; and in responseto determining that the external object is the wireless power receivingdevice, wirelessly transmitting power from the coil to the wirelesspower receiving device.
 11. The method of claim 10 wherein the wirelesspower receiving circuit resonant frequency is between 0.9 MHz and 1.1MHz and wherein monitoring the coil for presence of an external objectcomprises supplying the probe signal at a probe signal of less than 1.5MHz.
 12. The method of claim 11 wherein the oscillator is configured tosupply the probe signal to a node coupled to the coil and wherein thewireless power transmitting device includes an analog-to-digitalconverter configured to measure voltages on the node, the method furthercomprising processing output from the analog-to-digital converter todistinguish between thermal drift effects and object placement effects.13. The method of claim 12, wherein the coil is one of a plurality ofcoils and wherein monitoring the coil comprises supplying the probesignal to each of the plurality of coils in sequence.
 14. The method ofclaim 13 further comprising supplying the probe signal to the pluralityof coils in bursts separated by at least 50 milliseconds, wherein duringeach burst the oscillator supplies the probe signal to each of theplurality of coils in sequence.
 15. The method of claim 14 whereinprocessing the output from the analog-to-digital converter todistinguish between the thermal drift effects and the object placementeffects comprises comparing output signals from the analog-to-digitalconverter within a time window.
 16. The method of claim 12 whereinprocessing the output from the analog-to-digital converter todistinguish between the thermal drift effects and the object placementeffects comprises applying rate-of-change filtering to the output. 17.The method of claim 10, wherein monitoring the coil for presence of theexternal object comprises measuring a voltage of a node between theoscillator and the coil.
 18. The method of claim 17, wherein detectingthe external object comprises detecting a drop in the voltage of thenode between the oscillator and the coil.
 19. The method of claim 18,wherein obtaining the measurements to determine whether the externalobject is the wireless power receiving device comprises: using impulseresponse measurement circuitry, measuring decay of an impulse responsesignal that is produced in response to a pulse being applied to thecoil.
 20. The method of claim 10, wherein monitoring the coil forpresence of the external object by periodically supplying, with theoscillator, the probe signal to the coil at a probe frequency equal to101% to 150% of the wireless power receiving circuit resonant frequencycomprises periodically supplying, with the oscillator, the probe signalto the coil at a single probe frequency.